It is known that, in the case of ultrasonic transducers, an as high as possible ratio of wanted signal (acoustic energy via the measured medium) to disturbance signal (acoustic energy via the measuring tube) is desired. This ratio is also referred to as the signal to noise ratio or SNR. A large SNR is especially desired in the case of ultrasonic, inline, flow meters (FM) used for gaseous media, because, in such case, the in-coupling of the acoustic energy from the hard, solid body transducer (with high acoustic impedance) into the soft, gaseous medium (with low acoustic impedance) is especially inefficient, thus significantly less energy can be in-coupled. In order to be able to safely receive and evaluate the low acoustic energy at the oppositely lying transducer, it is of decisive importance that disturbance signals, which e.g. travel via the measuring tube walls, be kept away from the oppositely lying transducer. For this, a high SNR is decisive.
Known from the literature and patent documents are a large number of approaches to solutions for this goal. Among these are that proposed in German patent, DE19723488A1, in which the US (ultrasonic) transducer is provided on the periphery with a number of rubber protrusions, which enable engaging the US transducer acoustically decoupled from the measuring tube in a recess with peripheral groove. For preventing an acoustic short-circuit between US transducer and measuring tube recess, it is proposed to fill the remaining annular gap with “permanently elastic, closed pore, polyurethane foam”. A disadvantage of this solution is that, in the course of operating time of the US-FM, the polyurethane foam can change, such that it either loses its elastic properties or decomposes. In both cases, the acoustic decoupling effect is lost, leading to the result that the sound fractions traveling through solid structural elements, i.e. the structure-borne fractions, get continually stronger and, finally, an evaluation of the wanted signal is no longer possible.
Provided in European patent, EP1340964A1 is a geometry, which transfers radial oscillations of a sound producing plate via nested rings and sleeves, so-called filter elements, into a torsional deflection and thereby minimizes sound coupling into the housing attachment structure. A disadvantage of this solution is that the transducer housing is, due to the many filter elements, composed of a plurality of components, all of which must be connected durably sealed with one another. Moreover, the decoupling only functions against the axially recessed housing attachment: As soon as the annular gap between sound producing plate and transducer bore is shunted, be it by condensate or by solid deposits, the radial oscillation fractions are transmitted undamped to the measuring tube, and lost therewith is a safe evaluation of the wanted signal.
European Patent, EP2148322A2 discloses a US transducer housing, which likewise has structure-borne sound filter elements. The elements are, however, not placed directly against an axially and radially oscillating, sound producing plate, but, instead, placed a bit removed therefrom between such plate and a housing attachment to the measuring tube. The special feature of these filter elements is that at least two thereof are used, and that they have a resonant frequency matched to the sound production frequency. The disadvantages of this solution are identical to those relative to the geometry in European Patent, EP1340964A1.
Known from the state of the art is a relatively novel, generative manufacturing method, “selective laser melting”, from a 1999 dissertation (Wilhelm Meiners: Direktes Selektives Laser Sintern einkomponentiger metallischer Werkstoffe (Direct selective laser sintering of single component metal materials), Dissertation, RWTH Aachen 1999). It was then decisively further developed at the Fraunhofer-Institute für Lasertechnik (for laser technology) (ILT) in Aachen in cooperation with Dr. Matthias Fockele and Dr. Dieter Schwarze. This method is distinguished by characteristics that the material to be processed is applied in powder form in a thin layer on a platform and then locally completely melted by means of laser radiation, so that, after solidification, a fixed and medium excluding, material layer forms. Then, the platform is lowered by a coating thickness and powder applied anew. This cycle is repeated, until all layers are regrown. Typical layer thicknesses for all materials are 20-100 μm. In such case, applied materials are many and include a large number of metals and metal alloys. The data for guiding the laser beam are produced by means of software from a 3D CAD body. In order to prevent contamination of the material with oxygen, the process takes place under a protective gas atmosphere of argon or nitrogen.
Compared to conventional manufacturing methods, such as e.g. casting methods, laser melting is distinguished by features including that tools or forms are not used (formless manufacturing), so that the prototype construction time or product introduction time can be reduced. Compared to the CNC (Computer Numerical Control) processing of semifinished products, there is the advantage of an immense geometric freedom, which enables component forms, which are not, or only with great effort, manufacturable with conventional methods. Included in this connection are, for example, undercuts or partitions in the 20 μm thickness range.